An oral poliovirus vaccine trial in Costa Rica.

Early in the course of the 1959 nationwide oral poliovirus vaccination campaign in Costa Rica, a small, special and intensive study of oral vaccination was initiated there by the Yale Poliomyelitis Study Unit. The aims were to determine the effectiveness of vaccination, as measured by the rate of antibody conversions and antibody rises induced by the trivalent Lederle oral vaccine when given in two doses to a population of young children living in a tropical environment; to determine the prevalence of non-poliovirus enteroviruses in this juvenile population and their possible inhibitory effects upon the attenuated poliovirus vaccine strains; and to determine the duration of virus excretion by vaccinees and their young siblings, and the degree of intrafamilial spread of these viruses. It was also concerned with in vitro markers of the poliovirus strains recovered after one or two human passages; and, finally, dealt with the extent to which polioviruses could be recovered from flies trapped within the study area before and during the vaccination trial. Considerable knowledge was gained regarding the epidemiological problems which the use of oral vaccines entail.The authors conclude, inter alia, that the immunizing capacity of the vaccine used was "fairly good" and that its efficacy varied considerably for the three types of poliovirus. Type 3 poliovirus in the vaccine took well and also spread rapidly and well, infecting 85% of susceptible family contacts under 5 years old and reinfecting 25% of those who already had specific antibody. There was some evidence of interference with type 1 vaccine virus by other enteroviruses circulating at the time the first vaccine dose was given.